Understanding ARP

ARP provides IP communication within a Layer 2 broadcast domain by mapping an IP address to a MAC address. For example, host B wants to send information to host A but does not have the MAC address of host A in its ARP cache. In ARP terms, host B is the sender and host A is the target.

To get the MAC address of host A, host B generates a broadcast message for all hosts within the broadcast domain to obtain the MAC address associated with the IP address of host A. All hosts within the broadcast domain receive the ARP request, and host A responds with its MAC address.

Understanding ARP Spoofing Attacks

ARP spoofing attacks and ARP cache poisoning can occur because ARP allows a reply from a host even if an ARP request was not received. After the attack, all traffic from the device under attack flows through the attacker's computer and then to the router, switch, or host.

An ARP spoofing attack can affect hosts, switches, and routers connected to your Layer 2 network by sending false information to the ARP caches of the devices connected to the subnet. Sending false information to an ARP cache is known as ARP cache poisoning. Spoof attacks can also intercept traffic intended for other hosts on the subnet. Figure 15-1 shows an example of ARP cache poisoning.

Figure 15-1 ARP Cache Poisoning

Hosts A, B, and C are connected to the device on interfaces A, B, and C, all of which are on the same subnet. Their IP and MAC addresses are shown in parentheses; for example, host A uses IP address IA and MAC address MA. When host A needs to send IP data to host B, it broadcasts an ARP request for the MAC address associated with IP address IB. When the device and host B receive the ARP request, they populate their ARP caches with an ARP binding for a host with the IP address IA and a MAC address MA; for example, IP address IA is bound to MAC address MA. When host B responds, the device and host A populate their ARP caches with a binding for a host with the IP address IB and the MAC address MB.

Host C can poison the ARP caches of the device, host A, and host B by broadcasting two forged ARP responses with bindings: one for a host with an IP address of IA and a MAC address of MC and another for a host with the IP address of IB and a MAC address of MC. Host B and the device then use the MAC address MC as the destination MAC address for traffic intended for IA, which means that host C intercepts that traffic. Likewise, host A and the device use the MAC address MC as the destination MAC address for traffic intended for IB.

Because host C knows the true MAC addresses associated with IA and IB, it can forward the intercepted traffic to those hosts by using the correct MAC address as the destination. This topology, in which host C has inserted itself into the traffic stream from host A to host B, is an example of a man-in-the middle attack.

Understanding DAI and ARP Spoofing Attacks

DAI ensures that only valid ARP requests and responses are relayed. When DAI is enabled and properly configured, an NX-OS device performs these activities:

•Intercepts all ARP requests and responses on untrusted ports

•Verifies that each of these intercepted packets has a valid IP-to-MAC address binding before updating the local ARP cache or before forwarding the packet to the appropriate destination

•Drops invalid ARP packets

DAI can determine the validity of an ARP packet based on valid IP-to-MAC address bindings stored in a Dynamic Host Configuration Protocol (DHCP) snooping binding database. This database is built by DHCP snooping if DHCP snooping is enabled on the VLANs and on the device. It can also contain static entries that you create. If the ARP packet is received on a trusted interface, the device forwards the packet without any checks. On untrusted interfaces, the device forwards the packet only if it is valid.

You can configure DAI to drop ARP packets when the IP addresses in the packets are invalid or when the MAC addresses in the body of the ARP packets do not match the addresses specified in the Ethernet header (see the "Enabling or Disabling Additional Validation" section).

Interface Trust States and Network Security

DAI associates a trust state with each interface on the device. Packets that arrive on trusted interfaces bypass all DAI validation checks, and packets that arrive on untrusted interfaces go through the DAI validation process.

In a typical network configuration, the guidelines for configuring the trust state of interfaces as follows:

•Untrusted—Interfaces that are connected to hosts

•Trusted—Interfaces that are connected to devices

With this configuration, all ARP packets that enter the network from a device bypass the security check. No other validation is needed at any other place in the VLAN or in the network. For information about configuring the trust state of an interface, see the "Configuring the DAI Trust State of a Layer 2 Interface" section.

Caution Use the trust state configuration carefully. Configuring interfaces as untrusted when they should be trusted can result in a loss of connectivity.

In Figure 15-2, assume that both device A and device B are running DAI on the VLAN that includes host 1 and host 2. If host 1 and host 2 acquire their IP addresses from the DHCP server connected to device A, only device A binds the IP-to-MAC address of host 1. If the interface between device A and device B is untrusted, the ARP packets from host 1 are dropped by device B and connectivity between host 1 and host 2 is lost.

Figure 15-2 ARP Packet Validation on a VLAN Enabled for DAI

If you configure interfaces as trusted when they should be untrusted, you may open a security hole in a network. If device A is not running DAI, host 1 can easily poison the ARP cache of device B (and host 2, if you configured the link between the devices as trusted). This condition can occur even though device B is running DAI.

DAI ensures that hosts (on untrusted interfaces) connected to a device that runs DAI do not poison the ARP caches of other hosts in the network; however, DAI does not prevent hosts in other portions of the network from poisoning the caches of the hosts that are connected to a device that runs DAI.

If some devices in a VLAN run DAI and other devices do not, then the guidelines for configuring the trust state of interfaces on a device running DAI becomes the following:

•Untrusted—Interfaces that are connected to hosts or to devices that arenot running DAI

•Trusted—Interfaces that are connected to devices that are running DAI

To validate the bindings of packets from devices that are not running DAI, configure ARP ACLs on the device running DAI. When you cannot determine the bindings, isolate at Layer 3 the devices that run DAI from devices that do not run DAI. For configuration information, see the "Example 2: One Device Supports DAI" section.

Note Depending on your network setup, you may not be able to validate a given ARP packet on all devices in the VLAN.

Prioritizing ARP ACLs and DHCP Snooping Entries

When you apply an ARP ACL to traffic, the ARP ACLs take precedence over the default filtering behavior. The device first compares ARP packets to user-configured ARP ACLs. If the ARP ACL denies the ARP packet, the device denies the packet regardless of whether a valid IP-MAC binding exists in the DHCP snooping database.

Note VLAN ACLs (VACLs) take precedence over both ARP ACLs and DHCP snooping entries. For example, if you apply a VACL and an ARP ACL to a VLAN and you configured the VACL to act on ARP traffic, the device permits or denies ARP traffic as determined by the VACL, not the ARP ACL or DHCP snooping entries.

Logging DAI Packets

NX-OS maintains a buffer of log entries about DAI packets processed. Each log entry contains flow information, such as the receiving VLAN, the port number, the source and destination IP addresses, and the source and destination MAC addresses.

You can also specify the type of packets that are logged. By default, an NX-OS device logs only packets that DAI drops. For configuration information, see the "Configuring DAI Log Filtering" section.

Note NX-OS does not generate system messages about DAI packets that are logged.

Virtualization Support

The following information applies to DAI used in Virtual Device Contexts (VDCs):

•IP-MAC address bindings are unique per VDC.

•ARP ACLs are unique per VDC. You cannot use an ACL that you created in one VDC in a different VDC.

•Because ACLs are not shared by VDCs, you can reuse ACL names in different VDCs.

•The system does not limit ARP ACLs or rules on a per-VDC basis.

Licensing Requirements for DAI

The following table shows the licensing requirements for this feature:

Product

License Requirement

NX-OS

DAI requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the NX-OS licensing scheme, see the Cisco Nexus 7000 Series NX-OS Licensing Guide, Release 4.0.

Prerequisites for DAI

You should be familiar with the following before you configure DAI:

•ARP

•DHCP snooping

Guidelines and Limitations

DAI has the following configuration guidelines and limitations:

•DAI is an ingress security feature; it does not perform any egress checking.

•DAI is not effective for hosts connected to devices that do not support DAI or that do not have this feature enabled. Because man-in-the-middle attacks are limited to a single Layer 2 broadcast domain, you should separate the domain with DAI from domains without DAI. This separation secures the ARP caches of hosts in the domain with DAI.

•DAI depends on the entries in the DHCP snooping binding database to verify IP-to-MAC address bindings in incoming ARP requests and ARP responses. If you want DAI to use static IP-MAC address bindings to determine if ARP packets are valid, DHCP snooping needs only to be enabled. If you want DAI to use dynamic IP-MAC address bindings to determine if ARP packets are valid, DHCP snooping must configured on the same VLANs on which you configure DAI. For configuration information, see the "Configuring DHCP Snooping" section on page 14-6.

•When you use the feature dhcp command to enable the DHCP snooping feature, there is a delay of approximately 30 seconds before the I/O modules receive DHCP snooping or DAI configuration. This delay occurs regardless of the method that you use to change from a configuration with DHCP snooping disabled to a configuration with DHCP snooping enabled. For example, if you use the Rollback feature to revert to a configuration that enables DHCP snooping, the I/O modules receive DHCP snooping and DAI configuration approximately 30 seconds after you complete the rollback.

•When DHCP snooping is disabled or used in a non-DHCP environment, you should use ARP ACLs to permit or to deny packets.

•The DAI trust configuration of a port channel determines the trust state of all physical ports that you assign to the port channel. For example, if you have configured a physical port as a trusted interface and then you add that physical port to a port channel that is an untrusted interface, the physical port becomes untrusted.

•When you remove a physical port from a port channel, the physical port retains the DAI trust state configuration of the port channel.

•When you change the trust state on the port channel, the device configures a new trust state on all the physical ports that comprise the channel.

•If you want DAI to use static IP-MAC address bindings to determine if ARP packets are valid, ensure that the DHCP snooping feature is enabled and that you have configured the static IP-MAC address bindings. For configuration information, see the "Configuring DHCP Snooping" section on page 14-6.

DETAILED STEPS

Enables DAI for the specified list of VLANs. The no option disables DAI for the specified VLANs.

Step 3

showip arp inspection vlanlist

Example:

switch(config)# show ip arp inspection vlan 13

(Optional) Shows the DAI status for the specified list of VLANs.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Configuring the DAI Trust State of a Layer 2 Interface

You can configure the DAI interface trust state of a Layer 2 interface.

A device forwards ARP packets that it receives on a trusted Layer 2 interface but does not check them.

On untrusted interfaces, the device intercepts all ARP requests and responses, verifies that the intercepted packets have valid IP-MAC address bindings before updating the local cache and forwarding the packet to the appropriate destination. If the device determines that packets have invalid bindings, it drops the packets and logs them according to the logging configuration. For more information, see the "Configuring DAI Log Filtering" section.

(Optional) Copies the running configuration to the startup configuration.

Enabling or Disabling Additional Validation

You can enable or disable additional validation of ARP packets.

DAI intercepts, logs, and discards ARP packets with invalid IP-to-MAC address bindings. You can enable additional validation on the destination MAC address, the sender and target IP addresses, and the source MAC address.

BEFORE YOU BEGIN

By default, no additional validation of ARP packets is enabled.

SUMMARY STEPS

1. config t

2. [no] ip arp inspection validate {[src-mac] [dst-mac] [ip]}

3. show running-config dhcp

4. copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

switch(config)#

Enters global configuration mode.

Step 2

[no] ip arp inspection validate {[src-mac] [dst-mac] [ip]}

Example:

switch(config)# ip arp inspection validate src-mac dst-mac ip

Enables additional DAI validation, or if you use the no option, disables additional DAI validation.

Step 3

show running-config dhcp

Example:

switch(config)# show running-config dhcp

(Optional) Displays the DHCP snooping configuration, including the DAI configuration.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

The additional validations do the following:

•dst-mac—Checks the destination MAC address in the Ethernet header against the target MAC address in the ARP body for ARP responses. When enabled, packets with different MAC addresses are classified as invalid and are dropped.

•ip—Checks the ARP body for invalid and unexpected IP addresses. Addresses include 0.0.0.0, 255.255.255.255, and all IP multicast addresses. Sender IP addresses are checked in all ARP requests and responses, and target IP addresses are checked only in ARP responses.

•src-mac—Checks the source MAC address in the Ethernet header against the sender MAC address in the ARP body for ARP requests and responses. When enabled, packets with different MAC addresses are classified as invalid and are dropped.

When enabling additional validation, follow these guidelines:

•You must specify at least one of the keywords. You can specify one, two, or all three keywords.

•Each ip arp inspection validate command that you enter overrides the configuration from any previous commands. If you enter an ip arp inspection validate command to enable src-mac and dst-mac validations, and a second ip arp inspection validate command to enable IP validation only, the src-mac and dst-mac validations are disabled when you enter the second command.

Configuring the DAI Logging Buffer Size

You can configure the DAI logging buffer size.

BEFORE YOU BEGIN

The default buffer size is 32 messages.

SUMMARY STEPS

1. config t

2. [no] ip arp inspection log-buffer entriesnumber

3. show running-config dhcp

4. copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

switch(config)#

Enters global configuration mode.

Step 2

[no] ip arp inspection log-buffer entriesnumber

Example:

switch(config)# ip arp inspection log-buffer entries 64

Configures the DAI logging buffer size. The no option reverts to the default buffer size, which is 32 messages. The buffer size can be between 0 and 2048 messages.

Step 3

show running-config dhcp

Example:

switch(config)# show running-config dhcp

(Optional) Displays the DHCP snooping configuration, including the DAI configuration.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Configuring DAI Log Filtering

You can configure how the device determines whether to log a DAI packet.

Example Configurations for DAI

Example 1: Two Devices Support DAI

This procedure shows how to configure DAI when two devices support this feature. Host 1 is connected to device A, and Host 2 is connected to device B as shown in Figure 15-2. Both devices are running DAI on VLAN 1 where the hosts are located. A DHCP server is connected to device A. Both hosts acquire their IP addresses from the same DHCP server. Device A has the bindings for Host 1 and Host 2, and device B has the binding for Host 2. Device A Ethernet interface 2/3 is connected to the device B Ethernet interface 1/4.

Example 2: One Device Supports DAI

This procedure shows how to configure DAI when device B shown in Figure 15-2 does not support DAI or DHCP snooping.

If device B does not support DAI or DHCP snooping, configuring Ethernet interface 2/3 on device A as trusted creates a security hole because both device A and Host 1 could be attacked by either device B or Host 2.

To prevent this possibility, you must configure Ethernet interface 2/3 on device A as untrusted. To permit ARP packets from Host 2, you must set up an ARP ACL and apply it to VLAN 1. If the IP address of Host 2 is not static, which would make it impossible to accurately configure the ARP ACL on device A, you must separate device A from device B at Layer 3 and use a router to route packets between them.

To set up an ARP ACL on device A, follow these steps:

Step 1 Configure the access list to permit the IP address 10.0.0.1 and the MAC address 0001.0001.0001, and verify the configuration.

Configuring ARP ACLs

Session Manager Support

Session Manager supports the configuration of ARP ACLs. This feature allows you to create a configuration session and verify your ARP ACL configuration changes prior to committing them to the running configuration. For more information about Session Manager, see the Cisco Nexus 7000 Series NX-OS System Management Configuration Guide, Release 4.0.

Creating an ARP ACL

You can create an ARP ACL on the device and add rules to it.

BEFORE YOU BEGIN

Ensure that you are in the correct VDC (or use the switchto vdc command). ACL names can be repeated in different VDCs, so we recommend that you confirm which VDC you are working in.

Creates a rule that permits or denies any ARP message based upon the IP address and MAC address of the sender of the message. Using a sequence number allows you to specify a position for the rule in the ACL. Without a sequence number, the rule is added to the end of the rules.

Creates a rule that permits or denies ARP request messages based upon the IP address and MAC address of the sender of the message. Using a sequence number allows you to specify a position for the rule in the ACL. Without a sequence number, the rule is added to the end of the rules.

Creates a rule that permits or denies ARP response messages based upon the IPv4 address and MAC address of the sender and the target of the message. Using a sequence number allows you to specify a position for the rule in the ACL. Without a sequence number, the rule is added to the end of the rules.

Step 4

show arp access-lists acl-name

Example:

switch(config-arp-acl)# show arp access-lists arp-acl-01

(Optional) Shows the ARP ACL configuration.

Step 5

copy running-config startup-config

Example:

switch(config-arp-acl)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Changing an ARP ACL

You can add and remove rules in an existing ARP ACL. You cannot change existing rules. Instead, to change a rule, you can remove it and recreate it with the desired changes.

(Optional) Removes the rule that you specified from the ARP ACL. For more information about the permit and deny commands, see the "Creating an ARP ACL" section.

Step 5

showarpaccess-lists

Example:

switch(config-arp-acl)# show arp access-lists

Displays the ARP ACL configuration.

Step 6

copy running-config startup-config

Example:

switch(config-arp-acl)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Removing an ARP ACL

You can remove an ARP ACL from the device.

BEFORE YOU BEGIN

Ensure that you are in the correct VDC (or use the switchto vdc command). ACL names can be repeated in different VDCs, so we recommend that you confirm which VDC you are working in.

Ensure that you know whether the ACL is applied to a VLAN. The device allows you to remove ACLs that are currently applied. Removing an ACL does not affect the configuration of VLANs where you have applied the ACL. Instead, the device considers the removed ACL to be empty.

SUMMARY STEPS

1. config t

2. noarpaccess-listname

3. show arp access-lists

4. copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

switch(config)#

Enters global configuration mode.

Step 2

noarpaccess-listname

Example:

switch(config)# no arp access-list arp-acl-01

Removes the ARP ACL you specified by name from running configuration.

Step 3

showarpaccess-lists

Example:

switch(config)# show arp access-lists

Displays the ARP ACL configuration.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Changing Sequence Numbers in an ARP ACL

You can change all the sequence numbers assigned to rules in an ARP ACL.

BEFORE YOU BEGIN

Ensure that you are in the correct VDC (or use the switchto vdc command). ACL names can be repeated in different VDCs, so we recommend that you confirm which VDC you are working in.

SUMMARY STEPS

1. config t

2. resequence arp access-listname starting-sequence-number increment

3. show arp access-lists name

4. copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

switch(config)#

Enters global configuration mode.

Step 2

resequencearp access-list name starting-sequence-numberincrement

Example:

switch(config)# resequence arp access-list arp-acl-01 100 10

switch(config)#

Assigns sequence numbers to the rules contained in the ACL, where the first rule receives the starting sequence number that you specify. Each subsequent rule receives a number larger than the preceding rule. The difference in numbers is determined by the increment that you specify.

Step 3

showarpaccess-lists name

Example:

switch(config)# show arp access-lists arp-acl-01

Displays the ARP ACL configuration for the ACL specified by the name argument.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Copies the running configuration to the startup configuration.

Verifying ARP ACL Configuration

To display ARP ACL configuration information, use one of the following commands:

Command

Purpose

show arp access-lists

Displays the ARP ACL configuration.

show running-config aclmgr

Displays ACLs in the running configuration.

For detailed information about the fields in the output from these commands, see the Cisco Nexus 7000 Series NX-OS Security Command Reference, Release 4.0.